Undernourished prisoners of war at Stanley Internment Camp during the Japanese occupation of Hong Kong.

Sweet potatoes and tomatoes are among the many botanical introductions to maritime Asia to have resulted from the 16th-century Portuguese voyages of explora­tion. Along with sweetcorn, maize, guavas, papayas and, most notably, the chilli, these now commonplace food plants did not exist in this part of the world until Europeans introduced them.

Hardy and adaptable, they came into their own during the second world war, when imported foodstuffs were scarce.

“No rice to eat, eat sweet potatoes” was a common Cantonese refrain during the war years that is often quoted by elderly people nowadays. Widely grown as pig food, sweet potatoes remain unappealing for many who originally hailed from impoverished rural backgrounds. With the passage of time and rising prosperity, they felt that they could afford to eat something “better” and did not want dietary reminders of hard times and hostilities.

But, as 20th-century Chinese writer and philosopher Lin Yutang noted, patriotism is essentially nostalgia for the food of one’s childhood. Hong Kong’s burgeoning fondness for the humble sweet potato is one example of this simple truth and, roasted in charcoal, they are a popular street food, especially in the cooler months.

Also resurgent is a thin broth made of sweet potato chunks poached with old ginger and brown sugar, while in Taiwan, sweet potato leaves – commonly known as ground melon leaves – are popular as a healthy vegetable.

In early 1942, during the Japanese occu­pation of Hong Kong, canned American tomatoes were clandestinely obtained in Stanley Internment Camp. Whether the tins were sent in by friends or smuggled in on the black market is one of those minor historical details that were endlessly haggled over by ex-inmates, but whatever their provenance, these tomatoes were certainly eaten, and their residue – passed out “the other end” of those who consumed them – went into the communal compost.

And from that, one of Hong Kong’s minor wartime miracles followed, because tomato seeds can survive both canning processes and passage through the human digestive system to germinate in appropriate conditions. Credit for what came next belongs to Dr Geoffrey Herklots, a Hong Kong government botanist who was interned in Stanley.

Tomato seedlings were trans­planted into camp gardens and nourished with human fertiliser, the fruit subsequently gathered and fresh stocks replanted. By war’s end, in 1945, several tonnes of tomatoes had been cultivated on the playing fields of St Stephen’s College and in other garden patches around the camp.

Different ethnic groups in wartime Hong Kong were subject to different Japanese-issued ration cards. Europeans were assumed to be bread eaters, Indians were accorded an atta flour quota for making chapatis and Chinese were issued with rice.

Those who fell between these ethnicgroups, such as Eurasians, sometimes found it difficult to get any ration card at all in the early days of occupation, as various memoirs bitterly attest.

Preserved fish and fermented shrimp paste were two other wartime staples used across Asia. High in salt, these protein sources will keep almost indefinitely. The book Wartime Kitchen: Food and Eating in Singapore 1942-1950, by Wong Hong-suen, contains fascinating details of ingredients and recipes that were either common to – or completely different in – Japanese-occupied Singapore and Hong Kong.

A key difference between the two territories – and by extension, wider Southeast Asia and China – was that certain tropical plants were not grown in Hong Kong, and various palm-based sugars, such as gula Malacca, could not be produced so far north. This meant that occasional sweet dishes were possible for ordinary people in Singapore. In Hong Kong, however, imported cane sugar was subject to Japanese rationing and had become difficult to obtain by the time peace arrived.